Reversible redox of NADH and NAD+ at a hybrid lipid bilayer membrane using ubiquinone

Hybrid bilayer membranes as platforms for biomimicry and catalysis

Hybrid bilayer membrane (HBM) platforms represent an emerging nanoscale bio-inspired interface that has broad implications in energy catalysis and smart molecular devices. An HBM contains multiple modular components that include an underlying inorganic surface with a biological layer appended on top. The inorganic interface serves as a support with robust mechanical properties that can also be decorated with functional moieties, sensing units and catalytic active sites. The biological layer contains lipids and membrane-bound entities that facilitate or alter the activity and selectivity of the embedded functional motifs. With their structural complexity and functional flexibility, HBMs have been demonstrated to enhance catalytic turnover frequency and regulate product selectivity of the O₂ and CO₂ reduction reactions, which have applications in fuel cells and electrolysers. HBMs can also steer the mechanistic pathways of proton-coupled electron transfer (PCET) reactions of quinones and metal complexes by tuning electron and proton delivery rates. Beyond energy catalysis, HBMs have been equipped with enzyme mimics and membrane-bound redox agents to recapitulate natural energy transport chains. With channels and carriers incorporated, HBM sensors can quantify transmembrane events. This Review serves to summarize the major accomplishments achieved using HBMs in the past decade.

Thermodynamics regulated organic hydride/acid pairs as novel organic hydrogen reductants

Organic hydride/acid pairs could realize transformation of N-substituted organic hydrides from hydride reductants to thermodynamics regulated hydrogen reductants on conveniently choosing suitable organic hydrides and acids with various acidities.

Thermodynamics of the elementary steps of organic hydride chemistry determined in acetonitrile and their applications

This review focuses on the thermodynamics of the elementary step of 421 organic hydrides and unsaturated compounds releasing or accepting hydride or hydrogen determined in acetonitrile as well as their potential applications.

A highly active Cp*Ir complex with an anionic N,N-donor chelate ligand catalyzes the robust regeneration of NADH under physiological conditions

Anionic N,N-donor ligand chelate iridium complex [Cp*Ir(pba)Cl] 3 was developed, and exhibited the highest activity for NADH regeneration so far and stable chemoenzymatical coordinate catalytic performance for acetophenone enantioselective hydrogenation.

Surface modification of cerasomes with AuNPs@poly(ionic liquid)s for an enhanced stereo biomimetic membrane electrochemical platform

A novel liposomal nanocomposite, Au@PIL-cerasome, with biocompatibility and conductivity was fabricated via the self-assembly of cerasomes and gold nanoparticles (AuNPs) stabilized by poly(ionic liquid)s (PILs). The surface charge, morphology and chemical composition of the nanocomposites were characterized by the zeta potential, UV-vis, TEM, SEM and EDS. The nanocomposites exhibited structural stability directly on the surface of solid electrodes, without fusion. Electrochemical impedance experiments demonstrated that the nanocomposites had an enhanced conductivity compared with unmodified cerasomes. Horseradish peroxidase (HRP), as a reporter, was immobilized on the nanocomposites without denaturation or inactivation. The direct electron transfer of HRP was achieved, and the HRP/Au@PIL-cerasome/GCE exhibited an amplified current and improved electrocatalytic activity. Activity towards H₂O₂ displayed a linear range over 10-70 μM and a detection limit of 3.3 μM. Activity towards NO₂^- displayed linear ranges over 1-5 mM and 5-1280 mM, and the limit of detection was 0.11 mM. In addition, the electrode was stable and reproducible, with 6% RSD. Such multi-component liposomal nanocomposites with an enhanced electrical performance pave a better way for building novel and straightforward 3D stereo biomimetic electrochemical platforms and even molecular communication systems to investigate information transduction between cells.

Improvement of the Biocompatibility and Potential Stability of Chronically Implanted Electrodes Incorporating Coating Cell Membranes

The development of chronically implanted electrodes attracts much attention since these electrodes are much favorable for monitoring changes of neurotransmitters in brain science. The research in this field focused mainly on chemical modification to improve the potential stability and less on the biocompatibility. In this work, for the first time, we proposed the concept of cell-membrane electrodes based on a basic hypothesis using animal's self-cell membrane to reduce animal exclusiveness (hyperacute rejection and chronic rejection). As a proof of concept, we first studied cell-membrane reference electrodes for chronically implanted electrodes. Red cell membrane (RCM) was extracted from rat blood and coated on the chemically modified Ag/AgCl electrodes. It was found that ionic liquid (IL) 1-butyl-2,3-dimethylimidazolium hexafluorophosphate (BDMI) showed good performance rather than Nafion used as coating film for protection of silver chloride on Ag wire and support of the cell membrane. Electrochemical impedance spectra supported that charge-transfer resistance nearly kept constant before and after the electrodes were implanted into the rat's brain tissues for 28 days. Immunohistochemical analysis of the implant sites in the rat's brain tissues indicated that the extent of glial scarring arising from the Ag/AgCl/BDMI/RCM electrodes was smaller than that of both Ag/AgCl/Nafion and Ag/AgCl/Nafion/RCM electrodes after 28 days of implantation. The RCM-coated Ag/AgCl/IL electrodes showed a relatively potential stability compared to RCM-noncoated Ag/AgCl/IL electrodes after 28 days of implantation. Additionally, the current-voltage curve demonstrated that the RCM-coated electrodes can be used as polarized electrodes. This work demonstrated that the RCM, which was coated on the Ag/AgCl/IL electrodes, can significantly improve the biocompatibility and potential stability of the RCM-noncoated Ag/AgCl/IL electrodes implanted in the rat brain. The cell-membrane-coated electrodes will serve as a lighthouse in guiding the design of chronically implanted electrodes for in vivo electrochemical detection.

Investigation of the Ionization Mechanism of NAD+/NADH-Modified Gold Electrodes in ToF-SIMS Analysis

Analysis of nicotinamide adenine dinucleotide (NAD⁺/NADH)-modified electrodes is important for in vitro monitoring of key biological processes. In this work, time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to analyze NAD⁺/NADH-modified gold electrodes. Interestingly, no obvious characteristic peaks of nicotinamide fragment could be observed in the mass spectra of NAD⁺/NADH in their neutral sodium pyrophosphate form. However, after acidification, the characteristic peaks for both NAD⁺ and NADH were detected. This was due to the suppression effect of inner pyrophosphoric salts in both neutral molecules. Besides, it was proved that the suppression by inner salt was intramolecular. No obvious suppression was found between neighboring molecules. These results demonstrated the suppression effect of inner salts in ToF-SIMS analysis, providing useful evidence for the study of ToF-SIMS ionization mechanism of organic molecule-modified electrodes. Graphical Abstract ᅟ.

Transmembrane NADH Oxidation with Tetracyanoquinodimethane

The design of efficient schemes for nicotinamide adenine dinucleotide (NAD) regeneration is essential for the development of enzymatic biotechnological processes in order to sustain continuous production. In line with our motivation for the encapsulation of redox cascades in liposomes to serve as microbioreactors, we developed a straightforward strategy for the interfacial oxidation of entrapped NADH by ferricyanide as an external electron acceptor. Instead of the commonly applied enzymatic regeneration methods, we employed a hydrophobic redox shuttle embedded in the liposome bilayer. Tetracyanoquinodimethane (TCNQ) mediated electron transfer across the membrane and thus allowed us to shortcut and emulate part of the electron transfer chain functionality without the involvement of membrane proteins. To describe the experimental system, we developed a mathematical model which allowed for the determination of rate constants and exhibited handy predictive utility.

Colorimetric in situ assay of membrane-bound enzyme based on lipid bilayer inhibition of ion transport

Membrane-bound enzymes (MBEs), which make up a very high proportion of intracellular enzymes, catalyze a variety of activities that are currently analyzed by various techniques after purification. However, due to their amphipathic character, the purification of MBEs is difficult. Therefore, the most productive approach represents in situ analysis of MBEs in the cellular membrane. Methods: In this study, using membrane-bound α-glucosidase (α-Glu) as an example, we have developed a colorimetric in situ assay for MBEs based on the inhibitory effect of lipid bilayer on ion transport. The enzyme substrate could mediate the self-assembly of phospholipid PEG derivative around magnetic nanospheres that were modified with boronic acid. The formation of lipid bilayer could inhibit the leaking of iron ions under acidic conditions. However, the product of the catalytic reaction had no capability for self-assembly of the lipid bilayer, leading to the release of iron ions from the magnetic nanospheres under acidic pH. Results: The colorimetric in situ assay for MBEs could not only analyze the activity of membrane-bound α-Glu located on Caco-2 cells but could also evaluate the α-Glu inhibitors in cell medium. Conclusions: The simple, economic, and efficient method proposed here offers a potential application for high-throughput testing of α-Glu and its inhibitors. Our study also outlines a strategy for exploring the colorimetric method to detect the activities of MBEs in situ.

Reversible redox inter-conversion of biologically active NAD+/NADH derivatives bound to a gold electrode: ToF-SIMS evidence

The electrochemical reversible redox of NAD⁺/NADH is realized, which is confirmed by time-of-flight secondary ion mass spectrometry.

Stereodynamic Quinone-Hydroquinone Molecules That Enantiomerize at sp3-Carbon via Redox-Interconversion

Since the discovery of molecular chirality, nonsuperimposable mirror-image organic molecules have been found to be essential across biological and chemical processes and increasingly in materials science. Generally, carbon centers containing four different substituents are configurationally stable, unless bonds to the stereogenic carbon atom are broken and re-formed. Herein, we describe sp³-stereogenic carbon-bearing molecules that dynamically isomerize, interconverting between enantiomers without cleavage of a constituent bond, nor through remote functional group migration. The stereodynamic molecules were designed to contain a pair of redox-active substituents, quinone and hydroquinone groups, which allow the enantiomerization to occur via redox-interconversion. In the presence of an enantiopure host, these molecules undergo a deracemization process that allows observation of enantiomerically enriched compounds. This work reveals a fundamentally distinct enantiomerization pathway available to chiral compounds, coupling redox-interconversion to chirality.

Quantitative Detection of NADH Using a Novel Enzyme-Assisted Method Based on Surface-Enhanced Raman Scattering

An enzymatic method for quantitative detection of the reduced form of nicotinamide-adenine dinucleotide (NADH) using surface-enhanced Raman scattering was developed. Under the action of NADH oxidase and horseradish peroxidase, NADH can generate hydrogen peroxide (H₂O₂) in a 1:1 molar ratio, and the H₂O₂ can oxidize a chromogen into pigment with a 1:1 molar ratio. Therefore, the concentration of NADH can be determined by detecting the generated pigment. In our experiments, eight chromogens were studied, and o-tolidine (OT) was selected because of the unique Raman peaks displayed by its corresponding pigment. The optimal OT concentration was 2 × 10⁻³ M, and this gave the best linear relationship and the widest linear range between the logarithmic H₂O₂ concentration and the logarithmic integrated SERS intensity of the peak centered at 1448 cm⁻¹. Under this condition, the limit of detection for NADH was as low as 4 × 10⁻⁷ M. Two NADH samples with concentrations of 2 × 10⁻⁴ and 2 × 10⁻⁵ M were used to validate the linear relationship, and the logarithmic deviations were less than 3%.

Mimicking Photosynthesis with Electrode-Supported Lipid Nanoassemblies

The grand scale, ultimate efficiency, and sustainability of natural photosynthesis have inspired generations of researchers in biomimetic light energy utilization. As an essential and ubiquitous component in all photosynthetic machinery, lipids and their assemblies have long been recognized as powerful molecular scaffolds in building artificial photosynthetic systems. Model lipid bilayers, such as black lipid membranes and liposomes (vesicles), have been extensively used to host natural as well as synthetic photo- and redox-active species, thereby enabling key photosynthetic processes, such as energy transfer and photoinduced electron transfer, to be examined in well-defined, natural-like membrane settings. Despite their long history, these lipid models remain highly relevant and still enjoy wide practice today. In this Account, we share with the reader our recent effort of introducing electrode-supported lipid nanoassemblies as new lipid models into photosynthesis biomimicking. This line of research builds off several solid-supported lipid bilayer architectures established relatively recently by workers in membrane biophysics and reveals important new features that match and sometimes exceed what earlier lipid models are capable of offering. Here, our eight-year exploration unfolds in three sections: (1) New photosynthetic mimics based on solid-supported lipid bilayers. This systematic effort has brought three solid-supported bilayers into artificial photosynthesis research: lipid bilayers supported on indium tin oxide electrodes, hybrid bilayers, and tethered lipid bilayers formed on gold. Quantitative on-electrode deposition of various photo- and redox-active agents, including fullerene, Ru(bpy)₃²⁺, and porphyrin, is realized via liposomal hosts. Vectorial electron transfer across single lipid-bilayer leaflets is achieved between electron donor/acceptor directionally organized therein, taking advantage of multiple incorporation sites offered by these bilayers as well as their sequential formation on electrodes. Supported on electrodes, these bilayers uniformly afford reliable photocurrent generation and modular system design. (2) Gold-supported hybrid bilayers as a powerful model platform for probing biomembrane-associated photoelectrochemical processes. These hybrid nanostructures consist of one alkanethiol (or substituted alkanethiol) and one lipid monolayer, whose chemical identity and makeup can be separately controlled and modified. Such precise molecular organization and flexible formation, in turn, enable a series of physicochemical parameters key to photosynthetic processes to be explicitly examined and cross-compared. A few such examples, based on donor/acceptor distance and loading, interfacial dipole, and redox level, are included here to illustrate the usefulness and versatility of this system. (3) Mimicking photosynthesis with supercomplexed lipid nanoassemblies. This research effort was motivated to address the low light absorption suffered by single-bilayer based photosynthetic mimics and has yielded a new lipid-based approach to mimicking Nature's way of organizing multiple photosynthetic subunits. Rhodamine and fullerene assembled within these lipid supercomplexes display robust electronic communication. The remarkable possibility of using lipid matrix to further improve photoconversion efficiency is revealed by cholesterol, whose addition triggers exciton formation that promotes faster energy and electron transfer in these lipid nanoassemblies.

Bioinspired Assemblies and Plasmonic Interfaces for Electrochemical Biosensing

Electrochemical biosensing represents a collection of techniques that may be utilized for capture and detection of biomolecules in both simple and complex media. While the instrumentation and technological aspects play important roles in detection capabilities, the interfacial design aspects are of equal importance, and often, those inspired by nature produce the best results. This review highlights recent material designs, recognition schemes, and method developments as they relate to targeted electrochemical analysis for biological systems. This includes the design of electrodes functionalized with peptides, proteins, nucleic acids, and lipid membranes, along with nanoparticle mediated signal amplification mechanisms. The topic of hyphenated surface plasmon resonance assays is also discussed, as this technique may be performed concurrently with complementary and/or confirmatory measurements. Together, smart materials and experimental designs will continue to pave the way for complete biomolecular analyses of complex and technically challenging systems.

Efficient synthesis and antioxidant activities of N-heterocyclyl substituted Coenzyme Q analogues

A new strategy for the efficient synthesis of C-5 heterocyclyl substituted Coenzyme Q analogues was developed by N-alkylation of bromomethylated quinone 11 with a series of amines 12 under metal-free conditions. In vitro antioxidant activities of these Coenzyme Q analogues were evaluated and compared with commercial antioxidant Coenzyme Q10 by employing DPPH assay. All these N-heterocyclyl substituted Coenzyme Q analogues are found to be exhibiting good antioxidant properties and may be used as potent antioxidants for combating oxidative stress.

New artificial fluoro-cofactor of hydride transfer with novel fluorescence assay for redox biocatalysis

A new artificial fluoro-cofactor was developed for the replacement of natural cofactors NAD(P), exhibiting a high hydride transfer ability. More importantly, we established a new and fast screening method for the evaluation of the properties of artificial cofactors based on the fluorescence assay and visible color change.

Lipodisks integrated with weak affinity chromatography enable fragment screening of integral membrane proteins

Membrane proteins constitute the largest class of drug targets but they present many challenges in drug discovery.

Paper-Based Device for Rapid Visualization of NADH Based on Dissolution of Gold Nanoparticles

We describe a paper-based device that enables rapid and sensitive room-temperature detection of dihydronicotinamide adenine dinucleotide (NADH) via a colorimetric readout and demonstrate its value for monitoring NAD+-driven enzymatic reactions. Our system is based on NADH-mediated inhibition of gold nanoparticle (AuNPs) dissolution in a Au3+-cetyltrimethylammonium bromide (CTAB) solution. We fabricated a device consisting of a mixed cellulose ester paper featuring a wax-encircled, AuNP-coated film atop a cotton absorbent layer sandwiched between two plastic cover layers. In the absence of NADH, the Au3+-CTAB complex dissolves the AuNP layer completely, generating a white color in the test zone. In the presence of NADH, Au3+ is rapidly reduced to Au+, greatly decreasing the dissolution of AuNPs and yielding a red color that becomes stronger at increasing concentrations of NADH. This device exploits capillary force-assisted vertical diffusion, allowing us to apply a 25 μL sample to a surface-confined test zone to achieve a detection limit of 12.5 μM NADH. We used the enzyme glucose dehydrogenase as a model to demonstrate that our paper-based device can monitor NAD+-driven biochemical processes with and without selective dehydrogenase inhibitors by naked-eye observation within 4 min at room temperature in a small sample volume. We believe that our paper-based device could offer a valuable and low-cost analytical tool for monitoring NAD+-associated enzymatic reactions and screening for dehydrogenase inhibitors in a variety of testing contexts.

Monitoring Dopamine Quinone-Induced Dopaminergic Neurotoxicity Using Dopamine Functionalized Quantum Dots

Dopamine (DA) quinone-induced dopaminergic neurotoxicity is known to occur due to the interaction between DA quinone and cysteine (Cys) residue, and it may play an important a role in pathological processes associated with neurodegeneration. In this study, we monitored the interaction process of DA to form DA quinone and the subsequent Cys residue using dopamine functionalized quantum dots (QDs). The fluorescence (FL) of the QD bioconjugates changes as a function of the structure transformation during the interaction process, providing a potential FL tool for monitoring dopaminergic neurotoxicity.

pH triggered reversible photoinduced electron transfer to and from carbon nanoparticles

Dopamine functionalized carbon nanoparticles (CNPs) that can act as efficient photoinduced electron donor-acceptor systems depending on the pH of the medium have been synthesized. In acidic media, dopamine on CNPs exists as hydroquinone and serves as an electron donor while under alkaline conditions the corresponding quinone form of dopamine serves as a strong electron acceptor. Application of external NADH to the system can invert the donor-acceptor roles under alkaline conditions.

Biomimetic monolayer films of digalactosyldiacylglycerol incorporating plastoquinone

The photosynthesis is the process used by plants and bacteria cells to convert inorganic matter in organic thanks to the light energy. This process consist on several steps, being one of them the electronic transport from the photosystem II to the cytochrome thanks to plastoquinone-9 (PQ). Here we prepare membranes that mimic the characteristics and composition of natural photosynthetic cell membranes and we characterize them in order to obtain the PQ molecules position in the membrane and their electrochemical behaviour. The selected galactolipid is digalactosyldiacylglycerol (DGDG) that represents the 30% of the thylakoid membrane lipid content. The results obtained are worthful for several science fields due to the relevance of galactolipids as anti-algal, anti-viral, anti-tumor and anti-inflammatory agents and the antioxidant and free radical scavenger properties of prenylquinones. Both pure components (DGDG and PQ) and the DGDG:PQ mixtures have been studied using surface pressure-area isotherms. These isotherms give information about the film stability and indicate the thermodynamic behaviour of the mixture and their physical state. The Langmuir-Blodgett (LB) film has been transferred forming a monolayer that mimics the bottom layer of the biological membranes. This monolayer on mica has been topographically characterized using AFM and both the height and the physical state that they present have been obtained. Moreover, these monolayers have been transferred onto ITO that is a hydrophilic substrate with good optical and electrical features, so that, it is suitable for studying the electrochemical behaviour of these systems and it is a good candidate for energy producing devices.

A biomimetic platform to study the interactions of bioelectroactive molecules with lipid nanodomains

In this work, we developed a biomimetic platform where the study of membrane associated redox processes and high-resolution imaging of lipid nanodomains can be both performed, based on a new functional gold modification, l-cysteine self-assembled monolayer. This monolayer proved to be ideal for the preparation of defect-free planar supported lipid bilayers (SLBs) where nanodomains with height difference of ∼1.5 nm are clearly resolved by atomic force microscopy. Single and multicomponent lipid compositions were used, leading to the formation of different phases and domains mimicking the lateral organization of cellular membranes, and in all cases stable and continuous bilayers were obtained. These platforms were tested toward the interaction with bioelectroactive molecules, the antioxidant quercetin, and the hormone epinephrine. Despite the weak interaction detected between epinephrine and lipid bilayers, our biomimetic interface was able to sense the redox process of membrane-bound epinephrine, obtain its surface concentration (9.36 × 10(-11) mol/cm(2) for a fluid bilayer), and estimate a mole fraction membrane/water partition coefficient (Kp) from cyclic voltammetric measurements (1.13 × 10(4) for a fluid phase membrane). This Kp could be used to quantitatively describe the minute changes observed in the photophysical properties of epinephrine intrinsic fluorescence upon its interaction with liposome suspensions. Moreover, we showed that the lipid membrane stabilizes epinephrine structure, preventing its oxidation, which occurs in neutral aqueous solution, and that epinephrine partition and mobility in membranes depends on lipid phase, expanding our knowledge on hormone membrane interactions.

Facilitation of high-rate NADH electrocatalysis using electrochemically activated carbon materials

Electrochemical activation of glassy carbon, carbon paper and functionalized carbon nanotubes via high-applied-potential cyclic voltammetry leads to the formation of adsorbed, redox active functional groups and increased active surface area. Electrochemically activated carbon electrodes display enhanced activity toward nicotinamide adenine dinucleotide (NADH) oxidation, and more importantly, dramatically improved adsorption of bioelectrochemically active azine dyes. Adsorption of methylene green on an electroactivated carbon electrode yields a catalyst layer that is 1.8-fold more active toward NADH oxidation than an electrode prepared using electropolymerized methylene green. Stability studies using cyclic voltammetry indicate 70% activity retention after 4000 cycles. This work further facilitates the electrocatalysis of NADH oxidation for bioconversion, biosensor and bioenergy processes.

Enzyme nanoarchitectonics: organization and device application

Fabrication of ultrasmall functional machines and their integration within ultrasmall areas or volumes can be useful for creation of novel technologies. The ultimate goal of the development of ultrasmall machines and device systems is to construct functional structures where independent molecules operate as independent device components. To realize exotic functions, use of enzymes in device structures is an attractive solution because enzymes can be regarded as efficient machines possessing high reaction efficiencies and specificities and can operate even under ambient conditions. In this review, recent developments in enzyme immobilization for advanced functions including device applications are summarized from the viewpoint of micro/nano-level structural control, or nanoarchitectonics. Examples are roughly classified as organic soft matter, inorganic soft materials or integrated/organized media. Soft matter such as polymers and their hybrids provide a medium appropriate for entrapment and encapsulation of enzymes. In addition, self-immobilization based on self-assembly and array formation results in enzyme nanoarchitectures with soft functions. For the confinement of enzymes in nanospaces, hard inorganic mesoporous materials containing well-defined channels play an important role. Enzymes that are confined exhibit improved stability and controllable arrangement, which are useful for formation of functional relays and for their integration into artificial devices. Layer-by-layer assemblies as well as organized lipid assemblies such as Langmuir-Blodgett films are some of the best media for architecting controllable enzyme arrangements. The ultrathin forms of these films facilitate their connection with external devices such as electrodes and transistors. Artificial enzymes and enzyme-mimicking catalysts are finally briefly described as examples of enzyme functions involving non-biological materials. These systems may compensate for the drawbacks of natural enzymes, such as their instabilities under harsh conditions. We believe that enzymes and their mimics will be freely coupled, organized and integrated upon demand in near future technologies.

Investigation of an electrochemically switched heterocyclization reaction on gold surface

We report an investigation of an electrochemically switched heterocyclization reaction on hydroquinone-terminated self-assembled monolayers (SAMs). This reaction involves an electrochemically modulated hydroquinone/benzoquinone transformation step in the SAMs and a subsequent heterocyclization step taking place between the electrochemically generated benzoquinone moieties in SAMs and l-cysteine in solution. The reaction process was monitored by XPS and electrochemical surface-enhanced Raman spectroscopy (EC-SERS). The surface reaction proceeds as a two-step reaction to give a benzothiazine product, which is in contrast to the much more complicated multiple step reactions in solution. This result suggests that the tight molecular packing in the SAMs does not hinder the intramolecular heterocylization reaction, but prevents the intermolecular coupling reaction from happening. This work provides insights to the control and detection of biomolecule related multistep reactions occurring at solid-liquid interface.

Investigating electron-transfer processes using a biomimetic hybrid bilayer membrane system

Here we report a protocol to investigate the electron-transfer processes of redox-active biomolecules in biological membranes by electrochemistry using biomimetic hybrid bilayer membranes (HBMs) assembled on gold electrodes. Redox-active head groups, such as the ubiquinone moiety, are embedded in HBMs that contain target molecules, e.g., nicotinamide adenine dinucleotide (NADH). By using this approach, the electron-transfer processes between redox molecules and target biomolecules are mediated by mimicking the redox cycling processes in a natural membrane. Also included is a procedure for in situ surface-enhanced Raman scattering (SERS) to confirm the electrochemically induced conformational changes of the target biomolecules in the HBMs. In addition, each step in constructing the HBMs is characterized by electrochemical impedance spectroscopy (EIS), high-resolution X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The time required for the entire protocol is ∼12 h, whereas the electrochemical measurement of electron-transfer processes takes less than 1 h to complete.